A typical image sensor senses light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. After completion of integration cycle, charge is converted into a voltage, which is supplied to the output terminals of the sensor. In CMOS image sensors, the charge to voltage conversion is accomplished directly in the pixels themselves, and the analog pixel voltage is transferred to the output terminals through various pixel addressing and scanning schemes. The analog signal can be also converted on-chip to a digital equivalent before reaching the chip output. The pixels have incorporated in them a buffer amplifier, typically the source follower, which drives the sense lines that are connected to the pixels by suitable addressing transistors. After charge to voltage conversion is completed, and the resulting signal transferred out from the pixels, the pixels are reset in order to be ready for accumulation of new charge. In pixels that are using Floating Diffusion (FD) as the charge detection node, the reset is accomplished by turning on a reset transistor that momentarily conductively connects the FD node to a voltage reference. This step removes collected charge, however, it generates kTC-reset noise as is well known in the art. kTC noise has to be removed from the signal by a complicated Correlated Double Sampling (CDS) signal processing technique in order to achieve a desired low noise performance. The typical CMOS sensors that utilize the CDS concept need to have four transistors (4 T) in each pixel while the sensors without the CDS implementation need only 3 T. An example of the 4 T pixel circuit can be found in the U.S. Pat. No. 5,991,184 to Guidash. The description of the 3 T pixel concept can be found, for example, in: Fossum, E. R., “Active Pixel Sensors: Are CCD's Dinosaurs?,” Proceedings of the SPIE, vol. 1900, Charge-Coupled Devices and Solid State Optical Sensors III, Feb. 2-3, 1993, San Jose, Calif., pp. 2-14, and in Mendis, S., et al., “CMOS Active Pixel Image Sensor,” IEEE Transactions on Electron Devices, vol. 41, No. 3, March 1994, pp. 452-453.
The larger number of transistors in each pixel becomes a problem when the pixel size needs to be reduced in order to build low cost and high-resolution image sensors.
The disclosed invention addresses these difficulties and provides a simpler and practical solution for the 3 T pixel, which can be used in high-resolution compact size image sensors while having lower reset noise. By reducing the detection node capacitance during the reset cycle, the reset noise is also reduced. This is accomplished by forming a “parametric circuit” where the node capacitance depends on the node bias. The described circuit also maintains low dark current by using a shallow p-type implant placed close to the silicon-silicon dioxide interface to quench the interface states.